1. Field of the Invention
The present invention is directed to venting of direct vent combustible devices.
2. Description of the Related Art
Direct vent gas stoves and fireplaces are appliances that use a flue to vent combustion waste outside of a dwelling via the most direct route. Venting can occur either vertically, through a wall, generally referred to as a rear vent, or up through the ceiling, generally referred to as a top vent. The key advantage to direct vent appliances is that they are independent of room air and use their own combustion air. Direct vent units are pre-assembled in the factory, are usually made from metal and are made to be easy to install.
Perhaps the biggest advantage of a direct vent fireplace over a gas fireplace using a chimney is that the flue is much smaller in diameter than the average chimney. This means that less heat is lost through the flue than would be through a chimney.
In direct venting, room air is not used for combustion. Rather, air used for combustion is drawn into the combustion chamber by use of a vent which is exposed to the outside ambient air. A direct vent pipe includes two ducts formed by an inner pipe surrounded by a larger diameter outer pipe. A first duct connected to the vent conveys this outside air to the combustion chamber. After combustion, this air and the combustion byproducts are conveyed directly to the vent through a second duct which is isolated from the first duct. The two ducts are typically cylindrical and can be concentric, with the inlet air being conducted to the combustion chamber through an annulus outside the exit air duct and the exit air being conducted to the vent by way of the inner duct, co-linear (or side-by-side), or completely separate ducts.
Vent caps cover the inlet/outlet of the first and second ducts on the outside of a dwelling. One prior art vent typically used two flat plates located a distance away from the outlet of the exit duct. The inner flat plate, which is the plate closest to the exit duct, is impacted by the combustion exhaust products. Because it thereby became heated, a second or outer flat plate of virtually the same dimensions was separated a distance from the first plate to prevent burns. A third plate with a centre hole was provided between the first plate and the outlet of the exhaust duct. Another prior art cap is shown in U.S. Pat. No. 6,289,886. The cap shown therein utilizes a curved outer cover with an accurately shaped inner surface to dissipate exhaust gases. No “second plate” separates this accurately shaped surface from impact gasses.
Winds and drafts around the vent cap can also affect the backpressure in the duct. If there is backpressure present in the exit duct, the draw of inlet air will be reduced which will decrease combustion efficiency and can lead, in poorly designed systems, to extinguishing the combustion flame.